The present invention relates to a control circuit for a voltage inverter operable in an individual mode in which the inverter converts a DC voltage from a DC source, such as a fuel cell, a solar cell, or a battery, to an AC voltage, and individually transfers the converted AC voltage to an AC load, and in an interconnection mode, in which the inverter power system is interconnected with or incorporated into a power system, the converted AC voltage, together with the power voltage of the power system, is transferred to the load. More particularly, the present invention relates to a control circuit for the voltage inverter, which includes a compensating means for compensating an instantaneous variation of the power system voltage.
The control circuit for the voltage inverter operable in the individual and interconnection modes, comprises a constant voltage control system provided for use in the individual mode, and an effective power control system and a reactive power control system both provided for use in the interconnection mode. Generally, the inverter control circuit is provided with a synchronous-interconnection control system including phase and voltage amplitude adjusters, or an adjuster for adjusting those items of the output voltage. The synchronous-interconnection control system is used for a power system synchronous operation, which is performed when the inverter changes its operation mode from the individual mode to the interconnection mode.
The synchronous-interconnection control system, including the phase and voltage amplitude adjusters, stops those adjusters after the inverter power system is interconnected with the power system, and operates an effective power adjuster of the effective power control system and a reactive power adjuster of the reactive power control system. In the interconnection mode, the synchronous-interconnection control system and the effective and reactive power control systems are selectively changed over before and after the inverter power system is interconnected with the power system.
FIG. 4 is a block diagram showing a general circuit arrangement of the control circuit for voltage inverter operable in the individual and interconnection modes. In FIG. 4, reference numeral 1 designates a self-exciting voltage inverter; 2, an output reactor of the inverter 1; 3, a power system with which the inverter power system is interconnected; 4, a load; 5 a breaker for the interconnection mode; 6, a load breaker; 7, a voltage detect transformer (PT); and 8 a current detect transformer (CT).
An individual operation control system 10 includes a voltage adjuster 10B for adjusting a detection voltage Vid detected by a voltage detector 9 to approach to a value set by a voltage setter 10A. The control system applies the output voltage of the voltage adjuster 10B through a selector 42 to a pulse generation-amplifier (PA) 30, which is provided for applying a striking pulse to the invertor 1. The same applies a fixed frequency signal generated by an oscillator 11 through another selector 45 to the pulse generation-amplifier PA. Consequently, the inverter 1 transfers an AC power of which the reference frequency is the fixed frequency of the oscillator 11, to the load 4 through the load breaker 6.
An interconnected power system in which the power system of the inverter 1 is interconnected, for AC power transmission, with the power system 3, through the breaker 5, comprises an effective power control system 12 including a effective power adjuster 12B, and a reactive power control system 15 including a reactive power adjuster 15B. An effective-reactive power detector 14 receives and separates the AC power into an effective component P and a reactive component Q. The effective power adjuster 12B controls the output voltage of the inverter so as to make the detected effective component P approach to a set value set by an effective power setter 12A. The reactive power control system 15 controls the output voltage of the inverter so as to make the detected reactive component Q approach to a set value set by a reactive power setter 15A. A current/voltage (I/V) converter 31 converts the outputs of each control system into voltages corresponding to the output voltage of the inverter 1. The output voltages of the converter 31 are applied respectively through the selectors 41 and 42, to the pulse generation-amplifier (PA) 30. In this way, the inverter 1 is placed to the interconnection mode. A selector 45 is turned to a contact 45B so that the reference output frequency of the inverter 1 is controlled so as to be equal to the frequency of the power system 3.
A synchronous-interconnection control circuit 20, which is operated when the operation mode of the inverter is changed from the individual mode to the interconnection mode, is made up of the voltage detector 9, the voltage setter 10A, and the voltage adjuster 10B, which are common used with the individual control system 10. The output control signal of the voltage setter 10A is supplied through the contact 41B to the pulse generation-amplifier 30. A phase difference .phi. between the inverter output voltage and the power system voltage is detected by a phase difference detector 21. A phase-difference adjuster 20B, contained in the synchronous-interconnection control circuit, controls the reactive component of the inverter 1 so as to make the phase difference .phi. approach to a value that is set by a phase-difference setter 20A (normally, it is set at 0). The output signal of the phase-difference adjuster 20B is supplied to the pulse generation-amplifier 30, through the contact 42B of the selector 42.
In the individual mode, the breaker 5 is opened, and the respective selectors are turned to the contacts 41B, 42B, and 45A. Under this condition, an AC power of which, the voltage is controlled by the voltage adjuster 10B and the frequency is controlled by the oscillator 11, is supplied through the load breaker 6 to the load. The synchronous-interconnection control is performed in a manner that the selector 45 is turned to the contact 45B, the amplitude (instantaneous value) and the phase of the output voltage of the inverter 1 are made to approach to those values of the voltage of the power system 3, by the synchronous-interconnection control circuit 20, and thereafter, the breaker 5 is closed. The interconnection power system operation is performed by activating the effective power control system 12 and the reactive power control system 15 and turning the selectors to the contacts 41A and 42A.
Most of the interconnected power systems in which the inverter 1, which converts a DC voltage from a DC source, such as a fuel cell, a solar cell, or a battery, to an AC voltage, is operated while being interconnected with or incorporated into the power system, have each a relatively small power capacity. Because of this fact, an instantaneous variation of the voltage vector of the power system is large when the load is connected to and disconnected from the power system. The control circuit of the inverter, containing a proportional integration adjuster (PI adjuster) as a main component, is based on the fixed value control technique which operates to keep a set value at a fixed value against incoming disturbance. In the inverter control circuit, if, to improve the follow-up performance, the proportional compensation quantity is excessively increased by increasing the feedback quantity of the PI adjuster, the fixed value control becomes instable, so that the output of the inverter constantly varies.